1. Field of the Invention
The invention relates to the field of inductive thermal plasma torches and more precisely to the field of so-called “cold cage” torches.
2. Description of Related Art
An inductive thermal plasma torch uses a plasma gas, for example filtered or reconstituted air, which is injected at the base of the torch, it being possible for the reagents to be injected either at the base of the torch or at the tip of the torch. Compared with a conventional torch, having a water-cooled quartz chamber, the maximum output of which is approximately 150 kW, a cold-cage torch enables outputs of from 500 kW to 1 MW or thereabouts to be obtained. In addition, the life of a cold-cage torch is superior to that of a conventional quartz torch. The cold-cage torch has a confinement tube essentially composed of conductive confinement fingers that extend longitudinally in relation to the tube and are advantageously made of metal material, for example copper. The fingers are soldered at the base of the torch and are separated from one another by an insulating material, for example silica, in order to avoid the flow of induced currents in the confinement tube. The wall of the confinement tube is cooled, each finger preferably being cooled by an individual water circuit. The torch unit (torch base and confinement tube) is electrically insulated from the electrical ground of the whole plant that employs the torch, the said whole plant being, for example, a plasma recharge plant which uses silica particles of a primary preform to produce a fibre-optic final preform therefrom. When the torch is initiated, the plasma gas used is preferably not air, because the electric field required would be too great for direct initiation. Initiation is preferably carried out using an argon-type gas, which requires a smaller electric field and does not present a safety problem. However, the plasma obtained with argon is very short, because argon is a monoatomic gas. For that reason, once initiation is complete the argon is replaced, for example by air which, being an essentially diatomic gas, enables a longer and thus more effective plasma to be obtained for heating the preform, this longer plasma exhibiting an outer zone of appreciably higher energy than that of argon and allowing better thermal exchange with the preform.
The conventional ignition devices will not be suitable for cold-cage torches. Indeed conventional ignition devices will cause violent electrical arcs and destruction of certain parts or certain components of the cold-cage torch, or will be totally ineffective.
In a cold-cage torch, as in a conventional quartz torch, the electromagnetic field does not allow spontaneous ionisation of the plasma gas, which is why thermal or electrical initiation is necessary. A number of prior art ignition devices that enable initiation of a conventional quartz torch will be considered below.
According to a first prior art, the ignition device enabling initiation to be carried out is a Tesla coil. Coronas are created inside the torch and are converted to thermal plasma. However the Tesla coil creates an electric field that can pass through only a single thickness of an electrically transparent wall, such as, for example, the quartz wall of a conventional quartz torch. This ignition device has the drawback of being limited to conventional quartz torches, made of quartz and having a single thickness, that is to say, low-output torches.
According to a second prior art, the ignition is carried out by the generation of an electric arc, enabling the generation of coronas which will be converted to thermal plasma. Two ignition devices allow that outcome. The first ignition device is formed by two graphite electrodes connected to a power supply. Given that the use of graphite electrodes is complex and difficult, this first ignition device is complex and difficult and therefore cannot readily be used industrially. The second ignition device is formed by a coil of which the ends are opposite to each other and form a point discharge. A difference in potential is hence created between, on the one hand, one of the electrodes which is located at the end of the coil and, on the other hand, the other electrode which is linear and which passes through the interior of the coil. This second ignition device is either able to effect initiation of the plasma provided that a very high power is applied which, when the ends are well-shaped in the form of a point, may destroy the torch by the violence of the ignition or, when the ends are blunted and have become rounded, it may no longer function correctly and no longer cause initiation at all, which rapidly and inevitably occurs after several ignition attempts. This second device is consequently not very practical, especially for industrial applications.
According to a third prior art, the initiation is effected using cold plasma at low pressure. At low pressure, argon plasma is initiated spontaneously under the effect of a high-voltage and high-frequency electric field. Then, the vacuum is broken, and the temperature, the pressure and the power increase so as to be returned to the usual conditions of thermal plasma. This type of ignition device, however, requires a narrow chamber in which, for example, the movements of the primary preform which is being recharged may be so effected that the chemical reaction takes place in a controlled atmosphere. This third prior art, which proposes an actual “reactor”, is far too complex; it is unusable and oversized in relation to the need of the application, which is concerned merely with a “single flame” in the surrounding atmosphere.
According to a fourth prior art, the ignition device is formed by a refractory rod, for example made of graphite or molybdenum, inserted into the torch, which is then heated by induction so as to emit “thermal” electrons which will ionise the argon and thus initiate the discharge. For a conventional quartz torch, if the arm supporting the rod is electrically conductive, for example metallic, hence if the rod is connected to the electrical earth of the plant as a whole, initiation is effected under good conditions; conversely, if the arm supporting the rod is insulating, hence if the rod is of floating potential, insulated from the remainder of the plant, the initiation is achieved less effectively.
If it is desired, for a cold-cage torch, to use an ignition device according to the fourth prior art, the following two problems arise: either the arm supporting the rod is conductive, for example metallic, in which case the rod is connected to the electrical ground of the whole, and the initiation is effected but an electric arc is produced which damages the torch; or the arm supporting the rod is insulating, in which case the rod is insulated from the rest of the plant and then there is no initiation at all. On the one hand the essentially metal confinement tube appears to block the electric field of the inductor. On the other hand, the insulation between the fingers constituting the confinement tube prevents the flow of induced electric currents. Too little output, whether in the form of induced electric field or in the form of flow of induced currents, is therefore available in the torch and initiation is not successful.
There is known according to FIG. 3 of the American patent U.S. Pat. No. 5,017,751 a graphite rod 52 which is connected to the electrical ground (see lines 4–9, column 6), but that rod does not form part of the ignition device; it bas a totally different function which is that, during transition from the argon plasma to the nitrogen plasma, it enables degradation of the form of the plasma to be avoided (see lines 46–54, column 5 and lines 18–20, column 60).
There is known according to FIG. 1 of American patent U.S. Pat. No. 4,665,296 a rod Lb which is connected to the electrical ground, but no detail is given about the nature of the electric connection Lc connecting it to the electrical ground.